Jose Michel Tradition Brut and frankly their whole range is great and under $50 except for the Special Club. Although I believe Envoyer had that a few times for $40 or 50.
I have both Roederer brut premier and Pol Roger NV in the cellar and they do age indeed and can be found at <50$ (<35£) on special offers from time to time.
In fact, they release them as soon as previous lot has sold out and they are often onyl entering their drinking window. I typically give at least 2 years to my purchases in the cellar but often more.
As a general rule, I would think going from NV to vintage in this price range would mean getting lesser producers, which probably is not in your interest, whether aging wine or drinking it now.
Vilmart Grande Reserve NV is still in the mid $40s. It’s a richer style. I haven’t had this particular wine aged, but Vilmart generally ages very well.
Another recommendation. I would not start out buying 8 cases but rather would buy individual bottles of some of the recommendations here. Drink those, then get up to eight cases or whatever with your favorites. In your price range, there are a lot of good recommendations here but I also like Bereche and Marguet.
If you want one thing to go someone more expensive on for special occasions but still not go too expensive, try 2012 Delamotte for $80. It is fabulous and will age effortlessly for ages.
+1 on Maguet and Bereche. Laherte has a super compelling and reasonably priced range too.